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Understanding Metadata What is Metadata? .................................................................................................. 1 What Does Metadata Do? ................................................................................ 1 Structuring Metadata ................................................................................. 2 Metadata Schemes and Element Sets ............................................... 3 Dublin Core ..................................................................................................................... 3 TEI and METS .............................................................................................................. 4 MODS ....................................................................................................................... 5 EAD and LOM ...................................................................................................... 6 , ONIX, CDWA, and VRA .................................................................. 7 MPEG .......................................................................................................... 8 FGDC and DDI ........................................................................................ 9 Creating Metadata ................................................ 10 Interoperability and Exchange of Metadata ....11 Future Directions .................................... 12 More Information on Metadata ........ 13 Glossary ...................................... 15 Acknowledgements Understanding Metadata is a revision and expansion of Metadata Made Simpler: A guide for libraries published by NISO Press in 2001. NISO Press extends its thanks and appreciation to Rebecca Guenther and Jacqueline Radebaugh, staff members in the Library of Congress Network Development and MARC Standards Office, for sharing their expertise and contributing to this publication. About NISO NISO, a non-profit association accredited by the American National Standards Institute (ANSI), identifies, develops, maintains, and publishes technical standards to manage information in our changing and ever-more digital environment. NISO standards apply both traditional and new technologies to the full range of information-related needs, including retrieval, re-purposing, storage, metadata, and preservation. NISO Standards, information about NISO’s activities and membership are featured on the NISO website . This booklet is available for free on the NISO website ( and in hardcopy from NISO Press. Published by: NISO Press National Information Standards Organization 4733 Bethesda Avenue, Suite 300 Bethesda, MD 20814 USA Email: Tel: 301-654-2512 Fax: 301-654-1721 URL: Copyright © 2004 National Information Standards Organization ISBN: 1-880124-62-9 Understanding Metadata What Is Metadata? Metadata is structured information that describes, explains, locates, or otherwise makes it easier to retrieve, use, or manage an information resource. Metadata is often called data about data or information about information. The term metadata is used differently in different communities. Some use it to refer to machine understandable information, while others use it only for records that describe electronic resources. In the library environment, metadata is commonly used for any formal scheme of resource description, applying to any type of object, digital or non-digital. Traditional library cataloging is a form of metadata; MARC 21 and the rule sets used with it, such as AACR2, are metadata standards. Other metadata schemes have been developed to describe various types of textual and non-textual objects including published books, electronic documents, archival finding aids, art objects, educational and training materials, and scientific datasets. There are three main types of metadata: • Descriptive metadata describes a resource for purposes such as discovery and identification. It can include elements such as title, abstract, author, and keywords. • Structural metadata indicates how compound objects are put together, for example, how pages are ordered to form chapters. • Administrative metadata provides information to help manage a resource, such as when and how it was created, file type and other technical information, and who can access it. There are several subsets of administrative data; two that sometimes are listed as separate metadata types are: − Rights management metadata, which deals with intellectual property rights, and − Preservation metadata, which contains information needed to archive and preserve a resource. Metadata can describe resources at any level of aggregation. It can describe a collection, a single resource, or a component part of a larger resource (for example, a photograph in an article). Just as Metadata is key to ensuring that resources will survive and continue to be accessible into the future. catalogers make decisions about whether a catalog record should be created for a whole set of volumes or for each particular volume in the set, so the metadata creator makes similar decisions. Metadata can also be used for description at any level of the information model laid out in the IFLA (International Federation of Library Associations and Institutions) Functional Requirements for Bibliographic Records: work, expression, manifestation, or item. For example, a metadata record could describe a report, a particular edition of the report, or a specific copy of that edition of the report. Metadata can be embedded in a digital object or it can be stored separately. Metadata is often embedded in HTML documents and in the headers of image files. Storing metadata with the object it describes ensures the metadata will not be lost, obviates problems of linking between data and metadata, and helps ensure that the metadata and object will be updated together. However, it is impossible to embed metadata in some types of objects (for example, artifacts). Also, storing metadata separately can simplify the management of the metadata itself and facilitate search and retrieval. Therefore, metadata is commonly stored in a database system and linked to the objects described. What Does Metadata Do? An important reason for creating descriptive metadata is to facilitate discovery of relevant information. In addition to resource discovery, metadata can help organize electronic resources, facilitate interoperability and legacy resource integration, provide digital identification, and support archiving and preservation. Resource Discovery Metadata serves the same functions in resource discovery as good cataloging does by: • allowing resources to be found by relevant criteria; • identifying resources; • bringing similar resources together; • distinguishing dissimilar resources; and • giving location information. Organizing Electronic Resources As the number of Web-based resources grows exponentially, aggregate sites or portals are increasingly useful in organizing Page 1 links to resources based on audience or topic. Such lists can be built as static webpages, with the names and locations of the resources “hardcoded” in the HTML. However, it is more efficient and increasingly more common to build these pages dynamically from metadata stored in databases. Various software tools can be used to automatically extract and reformat the information for Web applications. Interoperability Describing a resource with metadata allows it to be understood by both humans and machines in ways that promote interoperability. Interoperability is the ability of multiple systems with different hardware and software platforms, data structures, and interfaces to exchange data with minimal loss of content and functionality. Using defined metadata schemes, shared transfer protocols, and crosswalks between schemes, resources across the network can be searched more seamlessly. Two approaches to interoperability are cross-system search and metadata harvesting. The Z39.50 protocol is commonly used for cross-system search. Z39.50 implementers do not share metadata but map their own search capabilities to a common set of search attributes. A contrasting approach taken by the Open Archives Initiative is for all data providers to translate their native metadata to a common core set of elements and expose this for harvesting. A search service provider then gathers the metadata into a consistent central index to allow cross-repository searching regardless of the metadata formats used by participating repositories. Digital Identification Most metadata schemes include elements such as standard numbers to uniquely identify the work or object to which the metadata refers. The location of a Page 2 digital object may also be given using a file name, URL (Uniform Resource Locator), or some more persistent identifier such as a PURL (Persistent URL) or DOI (Digital Object Identifier). Persistent identifiers are preferred because object locations often change, making the standard URL (and therefore the metadata record) invalid. In addition to the actual elements that point to the object, the metadata can be combined to act as a set of identifying data, differentiating one object from another for validation purposes. The latter group developed a framework outlining types of presentation metadata. A follow-up group, PREMIS (PREservation Metadata: Implementation Strategies)—also sponsored by OCLC and RLG—is developing a set of core elements and strategies for the encoding, storage, and management of preservation metadata within a digital preservation system. Many of these initiatives are based on or compatible with the ISO Reference Model for an Open Archival Information System (OAIS). Archiving and Preservation Structuring Metadata Most current metadata efforts center around the discovery of recently created resources. However, there is a growing concern that digital resources will not survive in usable form into the future. Digital information is fragile; it can be corrupted or altered, intentionally or unintentionally. It may become unusable as storage media and hardware and software technologies change. Format migration and perhaps emulation of current hardware and software behavior in future hardware and software platforms are strategies for overcoming these challenges. Metadata is key to ensuring that resources will survive and continue to be accessible into the future. Archiving and preservation require special elements to track the lineage of a digital object (where it came from and how it has changed over time), to detail its physical characteristics, and to document its behavior in order to emulate it on future technologies. Many organizations internationally have worked on defining metadata schemes for digital preservation, including the National Library of Australia, the British Cedars Project (CURL Exemplars in Digital Archives), and a joint Working Group of OCLC and the Research Libraries Group (RLG). Metadata schemes (also called schema) are sets of metadata elements designed for a specific purpose, such as describing a particular type of information resource. The definition or meaning of the elements themselves is known as the semantics of the scheme. The values given to metadata elements are the content. Metadata schemes generally specify names of elements and their semantics. Optionally, they may specify content rules for how content must be formulated (for example, how to identify the main title), representation rules for content (for example, capitalization rules), and allowable content values (for example, terms must be used from a specified controlled vocabulary). There may also be syntax rules for how the elements and their content should be encoded. A metadata scheme with no prescribed syntax rules is called syntax independent. Metadata can be encoded in any definable syntax. Many current metadata schemes use SGML (Standard Generalized Mark-up Language) or XML (Extensible Mark-up Language). XML, developed by the World Wide Web Consortium (W3C), is an extended form of HTML that allows for locally defined tag sets and the easy exchange of structured Understanding Metadata The Dublin Core Metadata Element Set arose from discussions at a 1995 workshop sponsored by OCLC and the National Center for Supercomputing Applications (NCSA). As the workshop was held in Dublin, Ohio, the element set was named the Dublin Core. The continuing development of the Dublin Core and related specifications is managed by the Dublin Core Metadata Initiative (DCMI). The original objective of the Dublin Core was to define a set of elements that could be used by authors to describe their own Web resources. Faced with a proliferation of electronic resources and the inability of the library profession to catalog all these resources, the goal was to define a few elements and some simple rules that could be applied by noncatalogers. The original 13 core elements were later increased to 15: Title, Creator, Subject, Description, Publisher, Contributor, Date, Type, Format, Identifier, Source, Language, Relation, Coverage, and Rights. The Dublin Core was developed to be simple and concise, and to describe Web-based documents. However, Dublin Core has been used with other types of materials and in applications demanding some complexity. There has for libraries is being developed by historically been some tension the Libraries Working Group. between supporters of a minimalist view, who emphasize the need to keep the elements Dublin Core Example to a minimum and the semantics and syntax Title=”Metadata Demystified” simple, and supporters of a structuralist view who Creator=”Brand, Amy” argue for finer semantic Creator=”Daly, Frank” distinctions and more extensibility for particular Creator=”Meyers, Barbara” communities. Subject=”metadata” These discussions have led to a distinction Description=”Presents an overview of metadata conventions in between qualified and publishing.” unqualified (or simple) Dublin Core. Qualifiers can Publisher=”NISO Press” be used to refine (narrow the scope of) an element, Publisher=”The Sheridan Press” or to identify the encoding Date=”2003-07" scheme used in representing an element value. Type=”Text” The element Date, for Format=”application/pdf” example, can be used with the refinement qualifier Identifier=” standards/resources/ created to narrow the Metadata_Demystified.pdf” meaning of the element to the date the object was Language=”en” created. Date can also be Because of its simplicity, the used with an encoding scheme qualifier to identify the format in Dublin Core element set is now which the date is recorded, for used by many outside the library example, following the ISO 8601 c o m m u n i t y — r e s e a r c h e r s , standard for representing date and museum curators, and music collectors to name only a few. There time. All Dublin Core elements are are hundreds of projects worldwide optional and all are repeatable. The that use the Dublin Core either for elements may be presented in any cataloging or to collect data from the order. While the Dublin Core Internet; more than 50 of these have description recommends the use of links on the DCMI website. The controlled values for fields where subjects range from cultural they are appropriate (for example, heritage and art to math and controlled vocabularies for the physics. Meanwhile the Dublin Core Subject field), this is not required. Metadata Initiative has expanded However, working groups have beyond simply maintaining the been established to discuss Dublin Core Metadata Element Set authoritative lists for certain into an organization that describes elements such as Resource Type. itself as “dedicated to promoting the While Dublin Core leaves content widespread adoption of interrules to the particular imple- operable metadata standards and mentation, the DCMI encourages developing specialized metadata the adoption of application profiles vocabularies for discovery (domain-specific rules) for particular systems.” domains such as education and government. An application profile Understanding Metadata Page 3 information. SGML is a superset of both HTML and XML and allows for the richest mark-up of a document. Useful XML tools are becoming widely available as XML plays an increasingly crucial role in the exchange of a variety of data on the Web. Metadata Schemes and Element Sets Many different metadata schemes are being developed in a variety of user environments and disciplines. Some of the most common ones are discussed in this section. Dublin Core The Text Encoding Initiative (TEI) The Text Encoding Initiative is an international project to develop guidelines for marking up electronic texts such as novels, plays, and poetry, primarily to support research in the humanities. In addition to specifying how to encode the text of a work, the TEI Guidelines for Electronic Text Encoding and Interchange also specify a header portion, embedded in the resource, that consists of metadata about the work. The TEI header, like the rest of the TEI, is defined as an SGML DTD (Document Type Definition)— a set of tags and rules defined in SGML syntax that describe the structure and elements of a document. This SGML mark-up becomes part of the electronic resource itself. Since the TEI DTD is rather large and complicated in order to apply to a vast range of texts and uses, a simpler subset of the DTD, known as TEI Lite, is commonly used in libraries. It is assumed that TEI-encoded texts are electronic versions of printed texts. Therefore the TEI Header can be used to record bibliographic information about both the electronic version of the text and about the non-electronic source version. The basic bibliographic information is similar to that recorded in library cataloging and can be mapped to and from MARC. However, there are also elements defined to record details about how the text was transcribed and edited, how mark-up was performed, what revisions were made, and other non-bibliographic facts. Libraries tend to use TEI headers when they have collections of SGML-encoded full text. Some libraries use TEI headers to derive MARC records for their catalogs, while others use MARC records as the basis for creating TEI header descriptions for the source texts. Page 4 Metadata Encoding and Transmission Standard (METS) an encoding format for metadata for textual and image-based works. The Digital Library Federation (DLF) built on that earlier work to create METS, a standard schema for providing a method for expressing and packaging together descriptive, administrative, and structural metadata for objects within a digital library. Expressed using the XML schema language, METS provides a document format for encoding the metadata necessary for management of digital library objects within a repository and for exchange between repositories. The Metadata Encoding and Transmission Standard (METS) was developed to fill the need for a standard data structure for describing complex digital library objects. METS is an XML Schema for creating XML document instances that express the structure of digital library objects, the associated descriptive and administrative metadata, and the names and locations of the files that comprise the digital object. The metadata necessary for successful Metadata in Action management and use of An oral historian makes tapedigital objects is both more recordings of interviews with members of extensive than and a particular ethnic group. Interviewees different from the sign a paper release form giving metadata used for intellectual property rights to the historian. managing collections of Most interviewees grant permission to printed works and other disseminate the interviews in print and physical materials. electronically, but several restrict publication and dissemination until 25 Structural metadata is years after death. needed to ensure that Information about each interview is separately digitized files kept in a database: Interviewer, (for example, different Interviewee, Date, Place, etc. Each pages of a digitized book) interview follows a questionnaire format. are structured appro- The questionnaire exists as a text file. The priately. Technical tapes, release forms, database, and text metadata is needed for file are donated to a library that has a information about the special collection focusing on the particular digitization process so ethnic group. The tapes are digitized. Since each that scholars may interview runs over several tapes, determine how accurate a technicians record structural metadata to reflection of the original keep component parts of each interview the digital version together. Technicians record provides. Other technical administrative metadata such as file metadata is required for names, location of each interview in the internal purposes in order files, equipment used, the methods of to periodically refresh and digitizing and assuring quality and migrate the data, ensuring completeness, file formats, etc. Different the durability of valuable segments of this metadata allow the audio files to be automatically tracked, accessed, resources. stored, refreshed, and migrated. METS was originally An archivist expands the database to an outgrowth of the include the persistent identifier of each Making of America II interview, thereby linking the audio file to project, a digitization the descriptive metadata. The names of project of major research the data elements are revised to match libraries that attempted to Dublin Core terminology, including address these metadata qualifiers used specifically for audio (continued on page 5) issues, in part by providing Understanding Metadata A METS document contains seven major sections: • METS Header – Contains metadata describing the METS document itself, including such information as creator, editor, etc. • Descriptive Metadata – Points to descriptive metadata external to the METS document (for example, a MARC record in an OPAC or an Encoded Archival Description finding aid maintained on a webserver), or to internally embedded descriptive metadata, or both. • Administrative Metadata – Provides information regarding how the files are created and stored, intellectual property rights, the original source object from which the digital library object derives, and the provenance of the files comprising the digital library object. • File Section – Lists all files containing content that comprise the electronic versions of the digital object. • Structural Map – Outlines a hierarchical structure for the digital library object and links the elements of that structure to content files and Metadata in Action (continued from page 4) metadata that pertain to materials. Information on rights and each element. permissions is entered. An archivist creates an EAD finding aid for the audio collection using the database as the core. Portions of the questionnaire text file are incorporated as a rich source of subject keywords. A MARC record is derived from the EAD finding aid and added to OCLC and RLIN. A webpage is created where researchers can access the finding aid, search the database, and listen to the audio files. Interviews coded as restricted are invisible to the search program until the date when they become open to the public. Administrative, structural, and descriptive metadata is created for the webpage to hold all the pieces together, allow them to be managed, and allow them to be accessed. The library participates in a metadata harvesting protocol to provide extracts of local metadata in a common format to a service provider so that information about the collection is automatically included in a number of relevant tools such as catalogs and portals. The webpage is linked to the library’s website dedicated to resources about the ethnic group, where it is available to researchers in context with archival and visual materials, digitized secondary sources, etc. Administrative, structural, and descriptive metadata at the website level has also been created. Understanding Metadata • called MIX, Metadata for Images in XML Schema, and is based on a proposed NISO standard, Z39.87, Data Dictionary: Technical Metadata for Digital Still Images. Further work is in process on extension schemas for audio, video, and websites. Another current area of concentration for the METS development community is the creation of METS application profiles to give guidance regarding the creation of METS documents for particular object types. Use of the METS schema is widespread. A list of implementation registries using METS, a tutorial, and other important information can be found on the METS website. Metadata Object Description Schema Structural Links – (MODS) Allows METS creators to record the nodes in the hierarchy outlined in the Structural Map. • Behavior – Associates executable behaviors with content in the METS object. The METS header, file section, structural map, structural links, and behavior sections are defined within the METS schema. METS is less prescriptive about descriptive and administrative metadata, relying on extension schemas— externally developed metadata schemes—to provide specific elements. The METS Editorial Board has endorsed three descriptive metadata schemes: simple Dublin Core, MARCXML, and MODS (discussed below). For technical metadata the METS website makes available schemas for text and digital still images. The latter standard is The Metadata Object Description Schema (MODS) is a descriptive metadata schema that is a derivative of MARC 21 and intended to either carry selected data from existing MARC 21 records or enable the creation of original resource description records. It includes a subset of MARC fields and uses languagebased tags rather than the numeric ones used in MARC 21 records. In some cases, it regroups elements from the MARC 21 bibliographic format. Like METS, MODS is expressed using the XML schema language. Although the MODS standard can stand on its own, it may also complement other metadata formats. Because of its flexibility and use of XML, MODS may potentially be used as a Z39.50 Next Generation specified format, an extension schema to METS, a metadata set for harvesting, and for creating original resource metadata records in an XML syntax. Rich description of electronic resources is a particular focus of MODS, which provides some advantages over other metadata Page 5 the EAD DTD provides support for both SGML and XML through the use of defined “switches” for Metadata demystified turning off features used only in SGML and turning on features used only in Brand XML. The EAD standard Amy is maintained jointly by the author Library of Congress and the Society of American Archivists. text The EAD is particularly 2003 popular in academic libraries, historical Bethesda, MD societies, and museums with large special NISO Press collections. Many of these 1-880124-59-9 collections contain unique materials unavailable elsewhere and often the materials in the schemes. MODS elements are description. Finding aids differ from richer than the Dublin Core; its catalog records by being much collections are not individually elements are more compatible with longer, more narrative and cataloged like traditional library library data than the ONIX or Dublin explanatory, and highly structured in materials. By creating searchable Core standards; and it is simpler to a hierarchical fashion. They EAD finding aids, libraries and apply than the full MARC 21 generally start with a description of archives can increase awareness of bibliographic format. With its use of the collection as a whole, indicating their unique collections to the XML Schema language, MODS what types of materials it contains Internet community. offers enhancements over MARC and why they are important. If the Learning Object Metadata 21, such as the use of an optional collection consists of the personal The IEEE Learning Technology ID attribute to facilitate linking at the papers of an individual there can be element level; the ability to specify a lengthy biography of that person. Standards Committee (LTSC) language, script, and transliteration The finding aid describes the series developed the Learning Object scheme at the element level; and into which the collection is Metadata (LOM) standard (IEEE the ability to embed a rich organized—such as corres- 1484.12.1-2002) to enable the use description of components in the pondence, business records, and re-use of technology-supported personal papers, and campaign learning resources such as related Item element. The ability in MODS to give speeches—and ends with an computer-based training and granular descriptions of constituent itemization of the contents of the distance learning. The LOM defines parts of an object works particularly physical boxes and folders the minimal set of attributes to manage, locate, and evaluate learning well with the METS structural map comprising the collection. for complex digital library objects. Like the TEI Header, the EAD is objects. The attributes are grouped defined as an SGML DTD. It begins into eight categories: A MODS Record Example The Encoded Archival Description (EAD) The Encoded Archival Description (EAD) was developed as a way of marking up the data contained in finding aids so that they can be searched and displayed online. In archives and special collections, the finding aid is an important tool for resource Page 6 with a header section that describes the finding aid itself (for example, who wrote it) and then goes on to the description of the collection as a whole and successively more detailed information about the records or series within the collection. If individual items being described exist in digital form, the EAD can include pointers to the digital objects. The 2002 version of • General, containing information about the object as a whole; • Lifecycle, containing metadata about the objects evolution; • Technical, with descriptions of the technical characteristics and requirements; • Educational, containing the educational / pedagogical attributes; Understanding Metadata • Rights, describing the intellectual allow property rights conditions; and use • Relation, identifying related objects; • Annotation, containing comments and the date and author of the comments; and • Classification, which identifies other classification system identifiers for the object. Within each category is a hierarchy of data elements to which the metadata values are assigned. Examples of learning-related metadata elements found in the Education category are Typical Age Range (of the intended user), Difficulty, Typical Learning Time, and Interactivity Level. The IMS Global Learning Consortium has developed a suite of specifications to enable interoperability in a learning environment. Their Meta-Data Information Model specification is based on the IEEE LOM scheme with only minor modifications. E-Commerce – and ONIX Metadata schemas are increasingly being developed to support electronic commerce applications. The Framework (Interoperability of Data in ECommerce Systems) was an international collaborative effort supported by the European Commission’s Info 2000 Programme. The collaborators were major rights owners, such as publishers and members of the recording industry, who wanted to develop a framework for metadata standards to support network commerce in intellectual property. The foundation of the work is a data model for intellectual property and its transfer. Rather than developing a new metadata scheme, sought to develop a common framework to Understanding Metadata various schemes for transactions related to different genres such as music, journal articles, and books to be able to interchange information, particularly that related to intellectual property rights. In order to support this common framework, has developed a minimal kernel of required metadata. Several organizations have built on the Framework to develop specific metadata schemas. Among them is the ONIX (Online Information Exchange) International standard. ONIX is an XML-based metadata scheme developed by publishers under the auspices of a number of book industry trade groups in the United States and Europe. The original ONIX specification was a direct response to the enormous growth in online book sales and the realization that books described with images, cover blurbs, reviews, and similar information significantly outsold books without this information. Therefore ONIX for Books has elements to record a wide range of evaluative and promotional information as well as basic bibliographic and trade data. ONIX for Serials is in development to define serials product metadata at the title, item, and subscription package levels. While ONIX information was designed for use in the commerce cycle of a publication, it may also provide a source for enrichment of library-created catalog records; the Bibliographic Enrichment Advisory Team (BEAT) project at the Library of Congress is experimenting with this use. ONIX metadata may also be used by libraries in the future for the creation of a beginning bibliographic record. Mappings between ONIX for Books and both MARC 21 and UNIMARC have already been created. Visual Objects – CDWA and VRA Metadata used to describe visual objects such as a painting or sculpture has its own special requirements. The Art Information Task Force (AITF), developed a conceptual framework for describing and accessing information about objects and images called Categories for the Descriptions of Works of Art (CDWA). Some 30 categories were defined, most with multiple subcategories. Some examples of the specialized descriptive elements relevant to artworks included are: Orientation, Dimensions, Condition, Inscriptions, Conservation Treatment, and Exhibition / Loan History. Typically, visual resources collections used in teaching art history and similar subjects do not contain original art works but rather slides or photographs of the original art. Metadata for these materials therefore has to accommodate the description of multiple levels of related resources, such as an original painting, a slide of the painting, and a digitized image of the slide. The VRA Core Categories build on and expand the CDWA work to define a single metadata element set that can be used to describe the work (the actual painting, photograph, sculpture, building, etc. ) as well as the images (visual representations) of them. Version 3.0 of the VRA Core Categories consists of 17 metadata elements which can be used as applicable to describe each of these versions and relate them to each other: Record Type, Type, Title, Measurements, Material, Technique, Creator, Date, Location, ID Number, Style/Period, Culture, Subject, Relation, Description, Source, and Rights. Like the Dublin Core, the VRA Core scheme does not specify any particular syntax or rules for representing content. Both CDWA and VRA emphasize the use of controlled vocabularies for specified elements. A number of existing vocabularies are suggested and communities are encouraged to develop additional vocabularies as needed. Page 7 MPEG Multimedia Metadata The ISO/IEC Moving Picture Experts Group (MPEG) has developed a suite of standards for coded representation of digital audio and video. Two of the standards address metadata: MPEG-7, Multimedia Content Description Interface (ISO/IEC 15938), and MPEG-21, Multimedia Framework (ISO/IEC 21000). MPEG-7 defines the metadata elements, structure, and relationships that are used to describe audiovisual objects including still pictures, graphics, 3D models, music, audio, speech, video, or multimedia collections. It is a multipart standard that addresses: • Description Tools including Descriptors that define the syntax and the semantics of each metadata element and Description Schemes that specify the structure and semantics of the relationships between the elements. • A Description Definition Language to define the syntax of the Description Tools, allow the creation of new Description Schemes, and allow the extension and modification of existing Description Schemes. • System tools, to support storage and transmission, synchronization of descriptions with content, and management and protection of intellectual property. Descriptors for visual and audio are defined separately using a hierarchy of elements and subelements. For visual objects there are descriptors for Basic Structure, Color, Texture, Shape, Motion, Localization, and Face Recognition. Audio descriptors are divided into two categories: low-level descriptors that are common to audio objects across most applications, and high-level descriptors that are specific to Page 8 particular applications of audio. The cross-application low-level descriptors cover Structures and Features (temporal and spectral). The domain-specific high-level descriptors include such elements as Musical Instrument Timbre, Melody Description, and Spoken Content Description. The Description Schemes are based on XML, and can be expressed in textual form suitable for editing, searching, filtering, and human readability; or in a binary form for storage, transmission, and streaming delivery. Since the full description of a multimedia object can be quite complex, the standard provides for a Summary Description Scheme geared to browsing and navigation. The standard envisions that search engines could use MPEG-7 metadata descriptions to identify audiovisual objects in entirely new ways, such as digitizing a musical phrase played on a keyboard and then retrieving a list of musical pieces that contain the sequence of notes; drawing some lines on an electronic drawing tablet and retrieving images with similar graphics; or using a voice excerpt to retrieve related speech files, photographs, video clips, and biographical information of the speaker. These retrieval mechanisms are outside the scope of MPEG-7, but the standards developers wanted to accommodate these futuristic capabilities and have included many interoperability requirements beyond the typical metadata elements. MPEG-21 was developed to address the need for an overarching framework to ensure interoperability of digital multimedia objects. The multi-part standard is not yet fully completed but is intended to include the following: • Part 1: Vision, Technologies and Strategy provides the overview of the complete vision and plan for the framework. It was issued as an ISO technical report (ISO/ IEC TR 21000:1-2001) and is available as a free download from ISO’s publicly available standards website. A second edition of the vision document is underway to address comments and suggestions received from other organizations following the initial publication. • Part 2: Digital Item Declaration, issued in 2003, describes a model for defining Digital Items. It includes a description of the syntax and semantics of each of the Digital Item Declaration elements and a corresponding XML schema. • Part 3: Digital Item Identification, also issued in 2003, describes how to uniquely identify Digital Items and how to link Digital Items with related information such as descriptive metadata. • Part 4: Intellectual Property Management and Protection is still in development. It is intended to define the framework for ensuring interoperability of intellectual property management tools, including authentication, and accommodates the Rights information defined in the following two parts. • Part 5: Rights Expression Language, issued in 2004, is a machine-readable language that can declare rights and permissions. • Part 6: Rights Data Dictionary is still in development. It will define a standard set of terms to be used with the Rights Expression Language. It is also expected to include specifications for mapping and transforming rights metadata terminology. The Rights Data Dictionary and Expression Language are being viewed as models for the handling of intellectual property metadata for applications beyond audiovisual. Understanding Metadata • Part 7: Digital Item Adaptation, also in development, is intended to standardize networking and interoperability description tools. Included in this part will be User Characteristic description tools that specify user preferences. There are some seven additional parts identified and in various stages of development that deal with technical interoperability issues of less specific relevance to metadata. All of the published parts are available from ISO as ISO/IEC 21000-[part#]. Documentation Initiative (DDI) information resources. The profile standard for describing social defines an extended set of data for science datasets. The DDI is describing biological data, such as defined as an XML DTD, and allows the taxonomic name of the for top down hierarchical description organism and its classification in the of a social science study, the data taxonomic hierarchy. files resulting from that Metadata in Action study, and the variables A county land planner is studying the used in the data files. There is also a header impact of new zoning laws on a particular area that uses Dublin Core bird species. The study team is composed elements for a high-level of an ecologist, hydrologist, civil engineer, and environmental protection specialist. description of the DDI Remote sensing data for the last 20 document itself. years provides a trend analysis of the decrease in wetlands, the bird’s habitat. These datasets have FGDC metadata. The biologists on the study team need to document the results of a field inventory. Using a biological profile to extend the FGDC element set, the biologists add the genus-species name and taxonomic hierarchy. The ecologists are concerned with collection methods and modeling tools. The data related to the changes in human population are documented using a metadata set developed by the Census Bureau. This study results in a technical report which is assigned Dublin Core metadata by the author. When the technical report is cataloged into the organization’s repository, the Dublin Core elements are used as the basis for automatic generation of a MARC cataloging record. This record is enhanced by the cataloger and included in the library’s online public access catalog. Metadata for Datasets Extensions and Profiles Metadata schemes for datasets are enabling original data in the science and social science fields to be shared in a way that was never possible before the Internet. One of the most well developed element sets is the Federal Geographic Data Committee (FGDC) Content Standard for Digital Geospatial Metadata (CSDGM), officially known as FGDC-STD-001-1998. Geospatial datasets include topographic and demographic data, GIS (geographic information systems), and computer-aided cartography base files. They are used in a wide variety of areas, including soil and land use studies, biodiversity counts, climatology and global change tracking, remote sensing, and satellite imagery. The FGDC Content Standard is required for use with resources created and funded by the U.S. Government and is also being used by many state governments. An international standard, ISO 19115, Geographic Information— metadata was issued in 2003. A technical amendment that will allow datasets to be both ISO and FGDC compliant is underway along with an implementation model that can be used in conjunction with an XML schema. A metadata scheme becoming well established in the social and behavioral sciences is the Data Despite the recent development of many of these metadata schemes, most have already been subject to the changes brought about by implementing them in real world situations. These modifications are of two types: extensions and profiles. An extension is the addition of elements to an already developed scheme to support the description of an information resource of a particular type or subject or to meet the needs of a particular interest group. Extensions increase the number of elements. Profiles are subsets of a scheme that are implemented by a particular interest group. Profiles can constrain the number of elements that will be used, refine element definitions to describe the specific types of resources more accurately, and specify values that an element can take. In practice, many applications use both extensions and profiles of base metadata schemes. For example, the National Biological Information Infrastructure (NBII) has developed a Biological Data Profile of the FGDC Content Standard for use with biological Understanding Metadata The U.S. Department of Education’s Gateway to Educational Materials (GEM) project has based their own metadata scheme on the Dublin Core. The GEM profile limits the Dublin Core elements that can be used (for example, Contributor is not allowed) and makes some elements mandatory. GEM also defines additional elements such as Audience, Grade, Quality, and Standards, extending the base Dublin Core set for educational use. Page 9 Creating Metadata Who creates metadata? The answer to this varies by discipline, the resource being described, the tools available, and the expected outcome, but it is almost always a cooperative effort. Much basic structural and administrative metadata is supplied by the technical staff who initially digitize or otherwise create the digital object, or is generated through an automated process. For descriptive metadata, it is best in some situations if the originator of the resource provides the information. This is particularly true in the documentation of scientific datasets where the originator has significant understanding of the rationale for the dataset and the uses to which it could be put, and for which there is little if any textual information from which an indexer could work. However, many projects have found that it is more efficient to have indexers or other information professionals create the descriptive metadata, because the authors or creators of the data do not have the time or the skills. In other cases, a combination of researcher and information professional is used. The researcher may create a skeleton, completing the elements that can be supplied most readily. Then results may be supplemented or reviewed by the information specialist for consistency and compliance with the schema syntax and local guidelines. Creation Tools Many metadata project initiatives have developed tools and made them available to others, sometimes for free. A growing number of commercial software tools are also becoming available. Creation tools fall into several categories: • Templates allow a user to enter the metadata values into pre-set fields that match the element set Page 10 being used. The template will then generate a formatted set of the element attributes and their corresponding values. • Mark-up tools will structure the metadata attributes and values into the specified schema language. Most of these tools generate XML or SGML Document Type Definitions (DTD). Some templates include such a mark-up as part of their final translation of the metadata. • Extraction tools will automatically create metadata from an analysis of the digital resource. These tools are generally limited to textual resources. The quality of the metadata extracted can vary significantly based on the tool’s algorithms as well as the content and structure of the source text. These tools should be considered as an aid to creating metadata. The resulting metadata should always be manually reviewed and edited. • Conversion tools will translate one metadata format to another. The similarity of elements in the source and target formats will affect how much additional editing and manual input of metadata may be required. Metadata tools are generally developed to support specific metadata schemas or element sets. The websites for the particular schema will frequently have links to relevant toolsets. Metadata Quality Control The creation of metadata automatically or by information originators who are not familiar with cataloging, indexing, or vocabulary control can create quality problems. Mandatory elements may be missing or used incorrectly. Schema syntax may have errors that prevent the metadata from being processed correctly. Metadata content terminology may be inconsistent, making it difficult to locate relevant information. The Framework of Guidance for Building Good Digital Collections, available on the NISO website, articulates six principles applying to good metadata: • Good metadata should be appropriate to the materials in the collection, users of the collection, and intended, current and likely use of the digital object. • Good metadata supports interoperability. • Good metadata uses standard controlled vocabularies to reflect the what, where, when and who of the content. • Good metadata includes a clear statement on the conditions and terms of use for the digital object. • Good metadata records are objects themselves and therefore should have the qualities of archivability, persistence, unique identification, etc. Good metadata should be authoritative and verifiable. • Good metadata supports the long-term management of objects in collections. There are a number of ongoing efforts for dealing with the metadata quality challenge: • Metadata creation tools are being improved with such features as templates, pick lists that limit the selection in a particular field, and improved validation rules. • Software interoperability programs that can automate the “crosswalk” between different schemas are continuously being developed and refined. • Content originators are being formally trained in understanding metadata and controlled vocabulary concepts and in the Understanding Metadata use of metadata-related software tools. • Existing controlled vocabularies Interoperability and Exchange of Metadata Some people ask: Do we need so many metadata standards? With all the metadata standards, initiatives, extensions, and profiles, how can interoperability be ensured? It is important to remember that different schemes serve distinct needs and audiences. Complementary schemes can be used to describe the same resource for • Communities of users are multiple purposes and to serve a developing and refining number of user groups. For exaudience-specific metadata ample, a technical report could have schemas, application profiles, a MARC metadata set in a library’s controlled vocabularies, and online catalog, an FGDC user guidelines. The MODS User description as part of the National Guidelines are a good example Spatial Data Infrastructure of the latter. Clearinghouse Mechanism, and an A Dublin Core description embedded set of represented in RDF Dublin Core ele
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